1. Field of the Invention
The invention relates to the field of intravascular imaging, in particular to an integrated ultrasound guided optical coherence tomography, photoacoustic probe used in intravascular or biomedical imaging and a method of using the same.
2. Description of the Prior Art
Intravascular ultrasound (IVUS) is a medical imaging methodology that has been used to show the anatomy of the wall of blood vessels in living animals and humans by using a miniaturized ultrasound probe. IVUS can help physicians determine the amount of plaque from the cross-sectional image of blood vessels. In other words, IVUS can visualize not only the lumen of the coronary arteries but also the objects hidden within the wall, such as atheroma. However, because the reflection coefficient of the ultrasound of blood vessel is quite small, high sensitivity and larger bandwidth ultrasound probe are key factors of high-quality intravascular ultrasound images. High sensitivity and large bandwidth probes can be fabricated by using high electromechanical coupling coefficient (Kt) piezoelectric materials. Research shows Pb(Mg1/3Nb2/3)O3—PbTiO3 (PMN-PT) is the one of the most promising high Kt commercial piezoelectric materials. It has been reported that a PMN-PT may be used as a single crystal transducer with a −6 dB fractional bandwidth of 114%.
On the other hand, the outer diameter of the ultrasound probe should be less than 3 mm to fulfill the requirement of IVUS biomedical imaging applications. Therefore, the fabrication of a miniaturized ultrasound probe is another important issue for IVUS imaging. High frequency (40 MHz) PMN-PT needle ultrasound transducers for biomedical applications have been made known in the art.
Optical coherence tomography (OCT) is a recently developed imaging modality using coherent gating to obtain high-resolution surface images of tissue microstructure. OCT endoscope design uses a fixed gradient-index (GRIN) lens and prism as the optical tip. Rotational torque is transferred from the endoscope's proximal end to the distal tip. OCT can provide imaging resolutions that approach those of conventional histopathology and can be performed in situ and in vivo. In vivo images of living animals have been demonstrated by using motor-based scanning endoscopic probes known in the art.
Nevertheless, one of drawbacks of OCT is that it needs to use saline water to flush blood away from the probe in order to remove the interference received from the blood. Therefore, how to minimize the times of saline water flushing is becoming a major topic in the OCT research filed nowadays. This problem is currently solved by inserting a balloon catheter at the imaging region to achieve blood occlusion, or by injecting relative large amounts of saline or other agents to flush away blood. However, both solutions have medical safety concerns. In the case of IVUS imaging, blood serves as the natural transmission media of the sound wave.
Additionally, the imaging resolution of IVUS is much less than that of OCT. In particular, IVUS is able to visualize the coronary artery from the inside-out owing to its larger penetration depth than OCT. In direct contrast, OCT can provide high-quality, micrometer-resolution, and three-dimensional images which are superior to IVUS.
Therefore, what is needed is a novel imaging probe combining a high frequency IVUS transducer with a 3-D scanning OCT probe to obtain the high-resolution cross-sectional intravascular images.
Optical coherent tomography (OCT) and ultrasound imaging are two of the most widely used image modalities. These image modalities share with common advantages, including: low-cost, high spatial resolution, portable, real-time, noninvasive, and non-radioactive. OCT and ultrasound imaging both measure cross-sectional tissue image. OCT measures tissue surface profile and cross-sectional image within a few millimeter depth range under the skin with a superior image resolution of 10 micrometers; high frequency ultrasound imaging also measures cross-sectional tissue image with a much deeper depth but with lower image resolution, on the order of 100 micrometers. OCT and ultrasound imaging modalities can be combined to provide a deeper cross-sectional imaging (tomography).
However, conventional ultrasound imaging performs relatively poor in blood vessel imaging, with lower imaging contrast, due to weak echo-genicity of blood. With recent developments in photoacoustics imaging, this limitation can be resolved. Photoacoustics imaging exploits the selective absorption property of hemoglobin to visible and near infrared (500-1200 nm) radiation, while tissues are relatively transparent in this optical spectrum. Through the optical absorption and thermoelastic expansion of blood vessels to short laser pulses, broadband ultrasound echo signals, up to 40 MHz, are generated from nanosecond laser radiated blood vessels. Since photoacoustic signals share the same acoustic spectra with ultrasound, photoacoustics imaging can be acquired and reconstructed by conventional ultrasound system.
A paper, entitled “Photoacoustic imaging of blood vessels with a double-ring sensor featuring a narrow angular aperture” by Kolkman et al. (Journal of Biomedical Optics, 9(6), 1327-1335, 2004) has proposed the development of a photoacoustic imaging probe, consisting of a double-ring polyvinylidene fluoride (PVDF) piezoelectric polymer sensor and an optical fiber located at its center. A 600 micrometer diameter optical fiber is used to transmit near infrared light to excite blood vessels; the double-ring piezoelectric polymer sensors acquire acoustic signal to generate ultrasound image.
U.S. Pat. No. 5,718,231, entitled “Laser ultrasound probe and ablator” describes a laser ultrasound probe, consisting of a ultrasound receiving sensor, made of PVDF piezoelectric polymer material for receiving photoacoustic signals and an optical fiber for transmitting laser radiation and generating photoacoustic signals by radiating the laser onto blood vessels.
Both of the above related prior art documents fail to present the concept of integrating OCT/ultrasound imaging/photoacoustics imaging modalities into a single image probe.